THE PRIMARY STAGES OF PHYLOGENY 293 



which are the ancestors of all animals. The ovum after the 

 nucleus had been re-formed became the cytula, which was 

 the ontogenetic counterpart of the amoeba. The morula, 

 a compact mulberry-like congeries of segmentation-cells, 

 corresponded to the synamceba, or earliest association of 

 undifferentiated amoeboid cells to form the first multi- 

 cellular organism. The blastula, or hollow sphere of seg- 

 mentation cells, usually ciliated, was reminiscent of the 

 plansea, an ancestral free-swimming form whose nearest 

 living relation is the spherical MagospJicera. The gastrula, 

 finally, is the two-layered sac formed from the blastula, 

 typically by invagination of its wall. It repeats the organisa- 

 tion of the gastraea, which is the common ancestor of all 



o ' 



Metazoa, and finds its nearest living counterpart in the 

 simple " sponges " Haliphysema and Gastrophyscina)- The 

 ancestral line of all the higher animals begins with the 

 five hypothetical forms of the moneron, amceba, synamceba, 

 plansea, and gastraea. 



We may take the following account 2 of the phylogeny 

 of the human species, from the gastraea stage onwards, 

 as typical of Haeckel's speculations on the evolution of 

 the higher forms. The progenitors of man are, after the 

 Gastraeada : 



i. Turbellaria. 

 *2. Scolecida. (Worms with a ccelom, probably represented 



at the present day by Balanoglossns.} 

 *3. Himatega. (Evolved from Scolecida by formation of 



dorsal nerve-tube and chorda, and resembling tailed 



larvae of Ascidians.) 



4. Acrania. (With metameric segmentation. Including 



Amphioxus.) 



5. Monorrhina. (Cyclostomes.) 



6. Selachia. 



7. Dipneusta. 



8. Sozobranchia. (Amphibia with permanent gills.) 



1 Studicn z. Gastraa-Theorie, p. 214, 1877. These forms were known 

 even in 1870 (Carter, Ann. Mag. Nat. Hist. (4), vi., pp. 346-7), to be 

 Foraminifera. The figures of supposed collar-cells, etc., do credit to 

 Haeckel's imagination. 



2 History of Creation, Eng. Trans., ii., pp. 278 ff. 



